This invention relates, in general, to Micro Electro-Mechanical System (MEMS) devices and, more particularly, to selective encapsulation in a MEMS device.
MEMS sensors, i.e. miniature sensors the size of an integrated circuit, have been introduced into a wide variety of consumer and industrial products that require small devices for sensing a variety of ambient conditions. Perhaps no current application has a greater potential for the incorporation of MEMS pressure sensors than automobile tires. The application requires the embedding of one or more MEMS pressure sensors, equipped with RF transmitters, within an automobile tire. The sensor would monitor tire pressure and transmit a signal providing a low pressure warning to the automobile""s processing and display devices, to alert drivers of an unsafe tire.
FIG. 1 illustrates a prior art technique of encapsulation of a MEMS device 10 in which an encapsulation gel 11 is injected into a cavity formed on a plastic package 12 that has a pressure sensor die 13 coupled to plastic package 12 via an adhesive 17. The purpose of encapsulation gel 11 was to protect wire bonds 15 between pressure sensor die 13 and lead frame 14. However, the total encapsulation technique previously used completely covered pressure sensor die 13, including pressure sensor diaphragm 16.
The primary drawback to this prior art total encapsulation gel technique is one of acceleration sensitivity. When encapsulation gel 11 covers pressure sensor diaphragm 16 and MEMS device 10 is placed in a tire that is subject to high revolutions per minute (RPM), the acceleration component realized by such a high RPM will force the mass of encapsulation gel 11 against pressure sensor 13, thereby providing a false tire pressure reading.
Another drawback to the total encapsulation gel technique is known as gel over expansion, commonly referred to as xe2x80x9cgel spewing.xe2x80x9d This phenomenon, caused by rapid changes in ambient pressure, results in the encapsulation gel being spewed or ejected from MEMS device 10. This in turn compromises the integrity of the device leading to contamination and ultimately premature device failure.
Another prior art approach to encapsulation of MEMS pressure sensors was known as conformal coating. This required a thin polymer to be applied over the pressure sensor die, wire bonds and lead frame. While partially solving the acceleration sensitivity problem, conformal coating had its own set of drawbacks.
In harsh environments, moisture would permeate the conformal coating causing delamination and peeling. Once the conformal coating has delaminated to the point of exposing the wire bonds, the MEMS device is subject to premature failure. Also, for parylene C coatings, heat in excess of 105 degrees Celsius (xc2x0 C) causes an oxidative degradation or other permanent material property change to the coating, thereby further subjecting the conformal coating to the problems of moisture.
Still another prior art encapsulation technique combined the total encapsulation gel with the conformal coating. Yet another technique was to encapsulate the MEMS device with a stainless steel cap or cover. However, these solutions were subject to the problems associated with acceleration sensitivity as well as higher cost.
Another prior art technique for manufacturing pressure sensor units is disclosed by Kobori et al. (xe2x80x9cKoborixe2x80x9d) in U.S. Pat. No. 4,802,952. Kobori discloses a wafer-level capping technique employing anodic bonding for piezoresistive elements. The disadvantage of Kobori is that the pressure sensitive diaphragms are formed by etching recesses on the underside of the silicon substrate. This technique adds cost and inefficiency to the manufacturing process.
Another prior art packaging technique is disclosed by Lam et al. (xe2x80x9cLamxe2x80x9d) in U.S. Pat. No. 4,942,383. Lam discloses a pressure sensor package where a die is adhesively coupled to a bottom cover and a top cover is coupled to the bottom cover by ultrasonic welding adhesive. Once again, the sensor diaphragm is located on the underside of the package. Furthermore, there is no preventative measure for precluding seepage of the die adhesive onto the diaphragm.
Still another prior art technique is disclosed in a pair of patents by Takahashi et al. in U.S. Pat. No. 5,207,102 (xe2x80x9cTakahashi ""102xe2x80x9d) and 5,333,505 (xe2x80x9cTakahashi ""505xe2x80x9d). In Takahashi ""102 a pressure sensor die is located on a top surface of a pedestal and the bottom surface of the pedestal is secured to a die pad. Wire bonding connects the pressure sensor die to leads. The die, wire bonds, pedestal and die pad are encapsulated by a conventional molding process using epoxy resin. Although not disclosed with particularity, it appears from the drawings and specification that Takahashi ""102 relies on temporary plugs as part of the molding process to limit the encapsulation of the epoxy resin.
Takahashi ""505 discloses an improved version of Takahashi ""102 having a resin dam surrounding the periphery of the piezoresistor to prevent a sheathing resin from flowing into the diaphragm during monolithic molding. A close reading of Takahashi ""505 reveals that the dam is actually a seal between the pressure sensor die and the molding plug used during epoxy resin encapsulation. The dam/seal helps to prevent the encapsulating resin from seeping onto the diaphragm as was apparently the drawback of Takahashi ""102. Takahashi ""505 discloses formation of the dam by a printing method or photoengraving. The height of the dam is approximately equal to the bond pads on the pressure sensor die. Note that the height of the molded encapsulation epoxy resin is significantly higher than that of the dam. Thus, the dam disclosed by Takahashi ""505 is a misnomer, i.e. it is not a true dam in that it does not create a cavity or reservoir, but rather is a seal to prevent resin seepage during the molding process.
Therefore, a need exists to provide a more reliable, improved quality and more robust encapsulation technique for protecting the integrity of MEMS sensors without incurring the performance handicaps and failure mechanisms of prior art techniques.